读懂react类组件中定义的static方法

react类组件中定义static方法有什么作用呢？

在java语言中也存在static方法，它属于类本身，跟对象实例没有关系，可以通过类名直接调用。react类组件中的static方法也是通过类名调用。

为什么使用静态方法：类内部的方法都会被子类继承，但是使用静态方法定义的不会被子类继承，也不会初始化到实例对象中。

static方法的本质是什么？

// index.jsx
import React from 'react'

export default class App extends React.Component {
  constructor() {
    super()
  }

  static say(msg) {
    console.log(msg)
  }

  render() {
    return <div>hello world.</div>
  }
}

使用babel7将上面代码转义到ES5，关于babel转义jsx可以参考我的另一篇文章react状态组件state定义在constructor里外的区别

"use strict";

Object.defineProperty(exports, "__esModule", {
  value: true
});
exports["default"] = void 0;

var _react = _interopRequireDefault(require("react"));

function _interopRequireDefault(obj) { return obj && obj.__esModule ? obj : { "default": obj }; }

function _typeof(obj) { "@babel/helpers - typeof"; if (typeof Symbol === "function" && typeof Symbol.iterator === "symbol") { _typeof = function _typeof(obj) { return typeof obj; }; } else { _typeof = function _typeof(obj) { return obj && typeof Symbol === "function" && obj.constructor === Symbol && obj !== Symbol.prototype ? "symbol" : typeof obj; }; } return _typeof(obj); }

function _classCallCheck(instance, Constructor) { if (!(instance instanceof Constructor)) { throw new TypeError("Cannot call a class as a function"); } }

function _defineProperties(target, props) { for (var i = 0; i < props.length; i++) { var descriptor = props[i]; descriptor.enumerable = descriptor.enumerable || false; descriptor.configurable = true; if ("value" in descriptor) descriptor.writable = true; Object.defineProperty(target, descriptor.key, descriptor); } }

function _createClass(Constructor, protoProps, staticProps) { if (protoProps) _defineProperties(Constructor.prototype, protoProps); if (staticProps) _defineProperties(Constructor, staticProps); return Constructor; }

function _inherits(subClass, superClass) { if (typeof superClass !== "function" && superClass !== null) { throw new TypeError("Super expression must either be null or a function"); } subClass.prototype = Object.create(superClass && superClass.prototype, { constructor: { value: subClass, writable: true, configurable: true } }); if (superClass) _setPrototypeOf(subClass, superClass); }

function _setPrototypeOf(o, p) { _setPrototypeOf = Object.setPrototypeOf || function _setPrototypeOf(o, p) { o.__proto__ = p; return o; }; return _setPrototypeOf(o, p); }

function _createSuper(Derived) { var hasNativeReflectConstruct = _isNativeReflectConstruct(); return function () { var Super = _getPrototypeOf(Derived), result; if (hasNativeReflectConstruct) { var NewTarget = _getPrototypeOf(this).constructor; result = Reflect.construct(Super, arguments, NewTarget); } else { result = Super.apply(this, arguments); } return _possibleConstructorReturn(this, result); }; }

function _possibleConstructorReturn(self, call) { if (call && (_typeof(call) === "object" || typeof call === "function")) { return call; } return _assertThisInitialized(self); }

function _assertThisInitialized(self) { if (self === void 0) { throw new ReferenceError("this hasn't been initialised - super() hasn't been called"); } return self; }

function _isNativeReflectConstruct() { if (typeof Reflect === "undefined" || !Reflect.construct) return false; if (Reflect.construct.sham) return false; if (typeof Proxy === "function") return true; try { Date.prototype.toString.call(Reflect.construct(Date, [], function () {})); return true; } catch (e) { return false; } }

function _getPrototypeOf(o) { _getPrototypeOf = Object.setPrototypeOf ? Object.getPrototypeOf : function _getPrototypeOf(o) { return o.__proto__ || Object.getPrototypeOf(o); }; return _getPrototypeOf(o); }

var App = /*#__PURE__*/function (_React$Component) {
  _inherits(App, _React$Component);

  var _super = _createSuper(App);

  function App() {
    _classCallCheck(this, App);

    return _super.call(this);
  }

  _createClass(App, [{
    key: "render",
    value: function render() {
      return /*#__PURE__*/_react["default"].createElement("div", null, "hello world.");
    }
  }], [{
    key: "say",
    value: function say(msg) {
      console.log(msg);
    }
  }]);

  return App;
}(_react["default"].Component);

exports["default"] = App;

从转义后代码可以看出_createClass(Constructor, protoProps, staticProps)第三个参数就是定义静态方法的，调用_defineProperties(Constructor, staticProps)方法将静态方法绑定到构造函数App(在js中App是一个函数对象)上。